1. Field of the Invention
The present invention relates to a process for the selective removal of hydrogen sulfide from gaseous streams, additionally containing carbon dioxide. More particularly, the present invention provides a process for selectively removing of hydrogen sulfide from gaseous streams containing hydrogen sulfide, even minor amounts of hydrogen sulfide, by oxidizing the hydrogen sulfide with a polyvalent metal chelate solution to form elemental sulfur. Additionally, the invention relates to the use of a particular type of aeration flotation apparatus to recover the formed sulfur particles from the metal chelate solution and to regenerate (oxidize) the reduced metal chelate.
2. Description of the Prior Art
The removal of hydrogen sulfide from gaseous streams, such as the waste gases liberated in the course of various chemical and industrial processes, for example, in the pulping of wood, natural gas and crude oil production and in petroleum refining, has become increasingly important in combating atmospheric pollution. Hydrogen sulfide-containing gases not only have an offensive odor, but such gases may cause damage to vegetation, painted surfaces and wildlife, as well as constitute a significant health hazard to humans. Government wide regulations have increasingly imposed continuously lower tolerances on the content of hydrogen sulfide which can be vented to the atmosphere, and it is now imperative in many localities to remove virtually all the hydrogen sulfide under the penalty of an absolute ban on continuing operation of a plant or the like which produces the hydrogen sulfide-containing gaseous stream.
The quantities of hydrogen sulfide in process gas streams are not very high. U.S. Pat. No. 3,071,433, dated Jan. 1, 1964 to Dunn, indicates that the stack gases obtained in the concentration of black liquor, the waste pulping liquor of the Kraft pulping process, contain from 500 to 2000 parts per million (ppm) of hydrogen sulfide. However, the odor of hydrogen sulfide can be detected by humans at a concentration of approximately 0.01 ppm. Consequently, an extremely efficient process for the removal of hydrogen sulfide is required to eliminate small amounts of noxious hydrogen sulfide from process gases.
Carbon dioxide may also be present with hydrogen sulfide as contaminants of gases, such as from well casings, combustion floods, geothermal steam, or tank vapors. Often, it is not only desirable to remove H.sub.2 S from such gases, but to selectively remove H.sub.2 S and not remove the carbon dioxide.
One well known method in the art for removing hydrogen sulfide from gas streams involves contacting the gas stream with caustic soda, which scrubs the acid gases from the gas stream. U.S. Pat. No. 2,747,962 to Heitz et al provides a method whereby acid gases, such as hydrogen sulfide, are removed selectively from a gas stream also containing carbon dioxide using an alkaline liquid, such as caustic soda, to remove the acid gas. The absorption of the CO.sub.2 is much slower than the absorption of H.sub.2 S, and thus the absorption of CO.sub.2 can be prevented by maintaining a very short contact time (0.01-0.02 second) between the gas stream and alkaline liquid. However, a disadvantage of this process is that when the alkaline liquid is regenerated by heating to around 270.degree. F., H.sub.2 S is produced, and thus the H.sub.2 S disposal problem is not solved but merely postponed.
It is also known to effect removal of hydrogen sulfide in an oxidation-reduction system by contacting the hydrogen sulfide-containing gas stream with a solution of a polyvalent cation (such as iron) complexed with a chelating agent (such as ethylenediaminetetraacetic acid or sodium salt thereof). In such a process, iron in the ferric state oxidizes the hydrogen sulfide to sulfur, the iron is reduced to the ferrous state, and the solution is regenerated by aeration to convert the iron back to the ferric state. The sulfur is recovered from the solution by froth flotation.
For example, U.S. Pat. No. 4,036,942 to Sibeud et al discloses a process for removing hydrogen sulfide and alkyl mercaptans from fluid stream by reaction with oxygen in the presence of a metal amino acid chelate in aqueous solution containing an amine, resulting in the conversion of hydrogen sulfide to sulfur and alkyl mercaptans to dialkyldisulfides, and separating these from the aqueous metal chelate solution. However, the presence of oxygen in the reactants is disadvantageous in that this results in the conversion of sulfur to sulfates and thiosulfates. Furthermore, the reaction requires a relatively long contact time between the metal chelate solution and the hydrogen sulfide-containing gas stream, such that if carbon dioxide is also present in the gas stream, the required contact time results in the absorption of carbon dioxide into the reaction solution, thus lowering the pH and lowering the efficiency of the reaction.
U.S. Pat. No. 4,009,251 to Meuly also discloses a process for removing hydrogen sulfide and alkyl mercaptans from gaseous streams by oxidizing the hydrogen sulfide to sulfur substantially without formation of sulfur oxides, in the presence of a metal chelate catalyst solution and a soluble alkali metal, or alkali earth metal or ammonium or amine salt of an acid having a pK within the range of about 1.2 to about 6. The alkyl mercaptans are oxidized to dialkyldisulfides under the same conditions. Meuly attempts to eliminate the oxidation of hydrogen sulfide through to sulfur oxides by the addition of the above-described acid salts. Such addition to the metal chelate catalyst solution is required, since Meuly reacts the hydrogen sulfide-containing gas stream with oxygen and recognizes that sulfur oxides may be formed by such a reaction mixture. Furthermore, the process of the above patent requires relatively long contact times for oxidation, and thus when carbon dioxide is present in the hydrogen sulfide-containing gas stream, the relatively long contact time also results in the absorption of CO.sub.2 and the consequential reduction in the pH of the solution and reduction in the efficiency of the system.